Gallium nitride crystal, which is attracting attention as a material for excellent blue light emitting element, is put into practical use in light emitting diode field, and further just beginning to be put into practical use also as an optical pick-up blue-violet semiconductor laser element. It is known to use MOCVD method, HVPE method, ammonothermal method, Na flux method and the like to perform growth of gallium nitride single crystal. For example, methods for growing gallium nitride single crystal by the Na flux method are disclosed in Japanese Patent Publication Nos. 2002-293696A and 2003-292400A.
On the other hand, the present applicants disclosed a method for efficiently growing gallium nitride single crystal under a specific condition by use of a hot isostatic press (HIP) apparatus in WO 2005/095682 A1.